Decide per-Schema predicates for the native RDF projection

[JeroenDeDauw]

For #1163

Q1 in NativeRdfProjection.md moves to Decided: predicates are scoped per Schema, $base/prop/{Schema}/{Property}. Later sharing across Schemas uses rdfs:subPropertyOf, and $base/prop/{Name} is reserved for name-level super-properties. Q10 no longer waits on Q1, and GlobalProperties.md no longer treats per-Schema anchoring as an argument in an open question.

The projection itself switches in #1163.

Considered, omitted:

• Updating the RDF Export reference and the Person → EDM guide: they describe what ships and change with the projection switch.
• Per-Schema prefixes and the range of a relation targeting several Schemas: implementation questions, kept in the issue.

_{AI-authored — Claude Code, Opus 5.5 (1M context) (max); asked for by @JeroenDeDauw after deciding Q1 in session, no revisions; diff not yet human-reviewed; docs only, line lengths and link anchors checked.}